The present invention relates to gas turbine engines, and more particularly, to a hardened liner disposed in the fan case of the engine to minimize damage in the event of a fan blade loss.
A gas turbine engine, such as a turbofan engine for an aircraft, includes a fan section, a compression section, a combustion section, and a turbine section. An axis of the engine is centrally disposed within the engine, and extends longitudinally through these sections. A primary flow path for working medium gases extends axially through the engine. A secondary flow path for working medium gases extends parallel to and radially outward of the primary flow path.
During operation, the fan draws air into the engine. The fan raises the pressure of the air drawn along the secondary flow path, thus producing useful thrust. The air drawn along the primary flow path into the compressor section is compressed. The compressed air is channeled to the combustor section, where fuel is added to the compressed air, and the air-fuel mixture is burned. The products of combustion are discharged to the turbine section. The turbine section extracts work from these products to power the fan and compressor. Any energy from the products of combustion not needed to drive the fan and compressor contributes to useful thrust.
The fan section includes a rotor assembly and a stator assembly. The rotor assembly of the fan includes a rotor disk and a plurality of outwardly extending rotor blades. Each rotor blade includes an airfoil portion, a root portion, and a tip portion. The airfoil portion extends through the flow path and interacts with the working medium gases to transfer energy between the rotor blade and working medium gases. The stator assembly includes a fan containment case assembly, which circumscribes the rotor assembly in close proximity to the tips of the rotor blades. The fan containment case assembly includes a fan case which provides a support structure, a plurality of fabric wraps disposed radially outwardly of the fan case, a plurality of circumferentially adjacent acoustic panels and a plurality of circumferentially adjacent rub strips disposed radially inwardly of the fan case. Conventional fan cases are typically a solid metal casing which forms a rigid structure to support the fabric wraps. The plurality of rub strips are formed from a relatively compliant material. In the event that the tip of a fan blade makes contact with the rub strips, the compliance of the rub strips minimizes the risks of damage to the fan blade.
It is desirable to a have reduced clearance between the fan blade tips and the fan case in turbine engines. There are two specific clearances between the fan blade tips and the fan containment case assembly which are of importance. The first one is characterized as a performance clearance and is defined as the clearance between the blade tips and the soft rub strip in the inner surface of the fan case. The second clearance is characterized as an effective structural clearance and is defined as the clearance between the blade tips and a hard metallic surface in the fan case. The present invention is concerned with this structural clearance, as opposed to the performance clearance.
The structural clearance between the hard surface of the fan case and the fan blade tips affects the dynamic response of the engine during severe rotor imbalance, particularly after a fan blade has failed and been released from the rotor assembly. A fan blade loss can result from either an impact with foreign objects or other structural reasons. The detached fan blade is thrown outward and passes through the fan case but is typically caught by the cloth wraps in the containment assembly. Blade loss produces an imbalance in the rotor and causes the rotor to move radially outward. The fan case then provides, in effect, a bearing surface to support the unbalanced array of fan blades. In this situation, the inner surface of the case acts as a bearing surface that engages the tips of the fan blades to support the rotor. The greater the initial radial separation between the fan blades and the inner surface of the case, the greater the amount of radial movement of the rotor that occurs before the case provides any bearing support. Movement of the rotor away from its longitudinal axis may also lead to additional damage to the rotor assembly. Minimizing the amount of radial movement minimizes the likelihood of further damage occurring. This decreased fan tip-to-case clearance reduces the imbalance sensitivity of the engine as the engine structure becomes xe2x80x9cstifferxe2x80x9d. However, due to their proximity to the fan case, the blades during a fan blade loss condition rapidly machine away the fan case because the blades are of usually a harder material than the fan case.
The fan blades with a tighter tip-to-case clearance, lean against the fan case with a much higher normal force to the fan case surface, thus creating a better structural load path. As a result, the engine""s overall sensitivity to the imbalance loads is reduced. On the other hand, the fan rotor must still turn. The blades with their increased normal force and harder material literally machine away the fan case and, more importantly, create very high drag forces on the perimeter of the fan case. This machining away of the fan case aggravates the high torque loads seen in every engine during a fan blade loss event. The high torque puts tremendous loads on the engine mounts and case structure. Thus, in order to reduce the sensitivity of the engine to rotating imbalances, a very high torque load results. The advantages of reducing the engine""s dynamic sensitivity to rotating imbalances are then lost to the generation of aggravated torque loads.
Thus, the challenge for modern gas turbine engines, during fan blade loss events, is the limiting of the rotor shaft deflection while minimizing the torque loading of the fan case from the rotor shaft kinetics.
According to the present invention, a fan case in a gas turbine engine includes a liner of hardened material attached thereto wherein during a fan blade loss condition, the blade tips skid on the hardened liner and reduce the destructive cutting away of the fan case. This liner of hardened material maintains the reduced fan-to-case clearances required to reduce the imbalance sensitivity of the gas turbine engine. Further, the sheet provides a skid-plate function which eliminates the generation of additional high torque loads due to the higher normal forces exerted by the fan blade tips while maintaining tight fan-to-case clearances. Further, the fan case structure of the present invention limits the deflection of the rotor shaft during a fan blade loss event. In one embodiment of the invention, the liner of hardened material comprises of shingles.
This invention is in part predicated on the recognition that by constraining the interaction of fan blade tips and the fan case to a predetermined radial zone in which is disposed hardened structure, there is a decrease of the loads transmitted to the interfaces of the engine by approximately the same percentage of the loads transmitted to the interfaces of the aircraft, and will allow an additional factor of safety during an abnormal imbalance condition of the rotor assembly.
According to one aspect of the present invention, a fan case in a gas turbine engine has a radial zone of interaction bounded outwardly by hard metallic surface of the fan case, the zone being a clearance which is less than one hundredth of the fan case diameter measured from the blade tips in a non-operative, zero speed engine condition with the rotor centered, a hardened structure disposed in the zone, such that during a high rotor imbalance condition, the blade tips skid on the hardened structure and reduce the destructive cutting away of the fan case, and reduce torque and imbalance loads transmitted to the interface of the engine and the aircraft.
In accordance with one particular embodiment of the invention, the optimal radial zone of clearance is defined as a constant approximately five-thousandths (0.005) of the fan case diameter.
In accordance with one particular embodiment of the invention, the lower limit of the radial zone of clearance is defined as a constant approximately two and one half thousandths (0.0025) of the fan case diameter, below which fan blades would destroy themselves due to high interaction loads between the fan blades and the fan case.
In accordance with another embodiment of the invention, the structural clearance lies in a range of 0.20 inches to 1.25 inches for corresponding jet engine fan case diameters which lie in a range of 20 inches to 120 inches.
The hardened structure or material is a liner which provides a skid-surface for the blades to circumferentially glide on and thus minimizes torque loading of the fan case. Further, the fan case structure of the present invention limits the deflection of the rotor shaft during a fan blade loss event. In one embodiment of the invention, the liner of the present invention comprises shingles of hardened material.
A primary advantage of the present invention is the minimization of damage to the fan case thus, resulting in a durable fan case in the event of a fan blade loss. The hardened fan case liner of the present invention reduces the destructive cutting away of the fan case by the fan blades. A further advantage is the maintenance of a minimum fan tip-to-case clearance which reduces the imbalance sensitivity of the engine. A further advantage of the fan case of the present invention is its ability to provide an appropriate restraining structure to the deflection of the rotor shaft during a fan blade loss event. In addition, the hardened liner reduces frictional forces and therefore, the torque transmitted from the rotor to the engine cases. Another advantage is the ease and cost of manufacturing and incorporating into the fan case the liner of the present invention. The simplicity of the structure of the liner and the use of economic materials, allows for cost effective manufacturing processes. Further, fan cases of the prior art can be retrofitted to include the present invention in a cost effective manner.